1. Field of Invention
This invention concerns a process and a system for removal of heavy metals from water or soil using metal tolerant microorganisms. In particular, this invention concerns a water treatment process and system for removal of metals from ground water or from soil by reducing or bioaccumulating the metals using metal tolerant Saccharomyces cerevisiae. The invention further concerns a process for bioreduction and bioaccumulation of the chromate using chromate tolerant Saccharomyces cerevisiae. The system and its basic functional flow diagram is provided which separates the metals from the water. The invention is particularly useful for removal or substantial reduction of levels of chromium, molybdenum, cobalt, zinc, nickel, calcium, strontium, mercury and copper in water.
2. Background Art and Related Art Disclosures
Metal-refining industries, mining operations, and manufacturing industries produce wastes that may contain metals. Occasional leaching of the waste solids by rain percolation or surface water runoff has been a source of contamination of the ground and surface water.
Of the metals, chromium is one of the most common contaminate. Chromium is used in plating shops, tanneries, and aerospace facilities, and has been found in the effluent of facilities with water towers and steam generators as well as in mine tailings. Epidemiological data suggest that chromium compounds are carcinogenic in humans (Br. J. Industr. Med., 13:260 (1956); Arch. Biochem. Biophys., 290:381 (1991). Studies described in Nature, 250:493 (1974) provide evidence that hexavalent chromium [Cr(VI)] salts, sodium, potassium and calcium are mutagenic.
The concentrations of hexavalent chromium is subject to ground water mandated discharge requirements. Hexavalent chromium Cr(VI) is monitored in ground water because it is toxic, and carcinogenic. It is also a very strong oxidizing agent. Cr(VI) is difficult to remove from ground water because of its high solubility at neutral pH. In its hexavalent form, chromium can be absorbed, sorbed or taken up by cells and cause toxic reactions. Chromium in the trivalent state is less toxic because at neutral pH it will precipitate as a hydroxide and will not be assimilated or taken up by cells. Because of its higher toxicity, the amount of hexavalent chromium is regulated. As set by the National Pollutant Discharge Elimination System, an allowable discharge limit for hexavalent chromium is only 11 ppb while trivalent chromium has an allowable discharge limit of 50 ppb.
Typical methods currently available for removing chromium are (1) chemical reduction with sodium metabisulfate, ferrous sulfate, or hydrogen peroxide; or (2) ion exchange. The ion-exchange method uses a polymer resin in which anions attach to the polymer and are exchanged with the Cr(VI) in solution. Potential problems associated with these methods include the cost of the resin and plugging of the resin-containing filter bed. Before the chemically treated water can be released at the surface, the sodium metabisulfate needs to be removed and the pH needs to be adjusted back to neutrality. Both method, are costly and contribute to additional toxic wastes.
It would, therefore, be highly advantageous to have available a method for removing or reducing chromium and other metals in ground water to levels established as acceptable by National Pollutant Discharge Elimination System (NPDES), utilizing a common nonpathogenic microorganism for such removal.
Several species of bacteria, yeast, and algae are known to be capable of accumulating metal ions extracellularly or internally to concentrations several orders of magnitude higher than the background concentration (J. Indust. Microbiol., 14:159 (1995). Pseudomonas species have been studied and characterized for chromate reductase activity as described in Appl. Environ. Microbiol., 56:2268 (1990). Although P. aeruginosa and P. fluorescens were shown successfully to remove Cr(VI) from ground water (unpublished results), these organisms also tainted the water with an unpleasant odor. Additionally, P. aeruginosa is a human pathogen and it is preferred not to use it for water treatment.
As described in J. Gen. Microbiol., 99:317(1977), and in Appl. Environ. Microbiol., 41:237 (1981), S. cerevisiae has been shown to be capable of accumulating cobalt (Co2+) and (Cd2+), cesium (Cs), strontium (Sr), and uranium (U). Copper [Cu(II)] and chromium (Cr2+) are also accumulated by S. cerevisiae (J. Ind. Microbiol., 7:97 (1991); Water Res., 24:433 (1990); Appl. Environ. Biotechnology, 41:149 (1994).
Microorganisms interact with metals by a number of processes, including transport, biosorption to cell biomass, entrapment in extracellular capsules, precipitation, and oxidation-reduction reactions as described, for example, in Experientia, 46:834 (1990) or in Metal Tolerance in Microbiology of Extreme Environments, Ed. C. Edwards, Open University Press, Milton Keynes, 178-210 (1990). Bioaccumulation of metal cations has been demonstrated by a process of an initial rapid accumulation that is independent of metabolism and temperature, and by a metabolically mediated process that internalizes the cation into the cell. These two processes are described in J. Gen. Microbiol, (supra) and Appl. Environ. Biotechnology (supra). Energy-dependent uptake of divalent cations by S. cerevisiae is described in Biochem. Biophys. Acta, 163:325 (1968) and in (J. Ind. Microbiol., 7:97 (1991) with influx being dependent on the electrochemical proton gradient across the plasma membrane as described ibid, at 650:88 (1981).
Removal of the metals from contaminated or polluted ground water or soil using the reduction or bioaccumulation of these metals by the metal tolerant microorganisms, including S. cerevisiae, has not been previously disclosed.
It is therefore a primary object of this invention to provide an effective and inexpensive process for removal of metals from ground water using the fermentative microorganism S. cerevisiae. 
All cited patents, patent applications and publications are hereby incorporated by reference.
One aspect of the current invention is a process for removal or a substantial reduction of metals from polluted ground water or from leached soil.
Another aspect of the current invention is a process for removal or a substantial reduction of metals from polluted ground water using yeast Saccharomyces cerevisiae. 
Still another aspect of the current invention is a process where the removal of a metal from water is achieved by bioreduction of the metal, or by accumulation and separation of the metal from the water using Saccharomyces cerevisiae.
Still yet another aspect of the current invention is a process for removal of metals from water utilizing Saccharomyces cerevisiae for accumulation and removal of the metals in a bioreactor which handles large volumes of water and separates chromium species accumulated in cells from water.
Yet another aspect of the current invention is a process for removal of toxic hexavalent chromium from the ground water by reducing the hexavalent chromium to trivalent chromium which is less toxic, can be precipitated as a hydroxide and is not assimilated by cells.
Still yet another aspect of the current invention is a process for removal of metals from water utilizing Saccharomyces cerevisiae for accumulation and removal of the metals in a bioreactor which handles large volumes of water and separates a metal accumulated in cells from water.
Still yet another aspect of the current invention is a process for removal of metals from the ground water wherein the metal is chromium, cobalt, copper, zinc, strontium, mercury, molybdenum or nickel.